Heat-sealing is a method of joining two materials under the action of pressure and heat wherein at least one of the substrates is provided with a thermoplastic coating (sealing medium) which undergoes plastic deformation at the sealing temperature and is firmly pressed to the other substrate under the sealing pressure applied. On cooling, the thermoplastic sealing medium solidifies, thus forming a strong adhesive bond between the two substrates.
Heat-sealing methods are largely employed for the packaging of goods, especially of foodstuffs. Illustrative of articles prepared in this manner are flat aluminum bags for dehydrated soups as well as yogurt, cups made of polystyrene and having covers of aluminum foil coated with a sealing varnish. Useful as sealing media are various thermoplastic resins (thermoplastic adhesives), e.g. copolymers of vinyl chloride, vinylidene chloride and vinyl acetate, polyacrylates as well as polymethacrylates such as polymethylmethacrylate, polyurethanes, thermoplastic polyesters, ethylene/vinyl acetate copolymers, aliphatic and aromatic polyamides, polyethylene, and ionomeric ethylene copolymers.
Heat-sealing of glass containers has been of little practical importance up to now. This may be attributed partly to the fact that the proper balance between temperature, time and pressure of the sealing operation is difficult to achieve, and partly to the formulation of the sealing medium itself. In addition, there are two specific problems which arise when glass is selected as one of the substrates to be heat-sealed.
On the one hand, glass does not undergo plastic deformation at the sealing temperatures employed so that the sealing medium is not pressed evenly onto the glass surface, especially since glass normally has surface irregularities (tolerances). Uniform sealing pressure is, however, indispensable for achieving a tight, impervious bond when the heat-sealing operation involves e.g. glass containers.
On the other hand, the glass surface is normally hydrated, i.e. covered with a thin water film that cannot be completely removed even by high-vacuum heating to a temperature of 200.degree. C. for a period of several hours. For the heat-sealing of glass, this means bonding the sealing medium to a surface covered with water molecules. Thus, all heat-sealing bonds obtainable with conventional sealing media on untreated glass containers substantially lose their strength within several hours or a few days. This is especially true when the heat-sealed substrates are exposed to moisture, as in the case of containers which are either filled with goods containing water, or are stored in water. It is assumed that in these instances, water molecules migrate under the sealing medium, aided by the above described water film.
German Auslegeschrift No. 28 33 334 discloses a method of improving the moisture resistance of adhesive bonds between glass and metal foils by first coating the rim of a glass container with an adhesion promoter having high affinity for glass as well as available reactive sites for crosslinking with the adhesive layer. As suitable adhesion promoters, there are mentioned silanes and organosilicon compounds, the latter not being specified in detail.
The use of such adhesion promoters for the heat-sealing of glass containers has the disadvantage that additional process steps and equipment are required for applying the adhesion promoters. This operation is less economical as investments will be considerably higher while the rates of series production are slowed down. Moreover, when proceeding in accordance with the prior art method of heat-sealing glass containers, there will always be the risk of food packed in these containers becoming contaminated by the adhesion promoter.
Thus, it is an object of the present invention to provide a method of heat-sealing substrates, particularly glass to glass or to other materials, which results in heat-sealing bonds of high moisture resistance without the use of an adhesion promoter for glass.
It has now been found that strong, durable heat-sealing bonds may be obtained by employing heat-sealing adhesives on the basis of thermoplastic, organomodified silicate materials (silicic acid heteropolycondensates) instead of conventional thermoplastic adhesives. When one of the substrates to be heat-sealed is glass, the sealing bond shows excellent moisture resistance even if the glass surface has not been previously coated with an adhesion promotor.